Recently, with accelerated upsizing and high definition of liquid crystal display devices (LCDs), liquid crystal display devices have been increasingly becoming common in applications that mainly display static images, like liquid crystal display devices for use in personal computers, word processors, and the like and applications that display moving images, like liquid crystal display devices for use in TV and the like. Liquid crystal display devices are increasingly becoming common in general households. This is because a liquid crystal display device is thinner and takes up less space than TV including a cathode ray tube (hereinafter referred to as CRT).
However, a liquid crystal display device has an optical response speed slower than CRT (Cathode-Ray Tube) and others, and may not complete a response within a rewrite time (16.7 msec) corresponding to a usual frame frequency (60 Hz), depending upon grayscale transition. For example, Patent document 1 (Japanese Unexamined Patent Publication No. 365094/1992 (Tokukaihei 4-365094); published on Dec. 17, 1992) adopts a technique of driving with a drive signal modulated so as to emphasize grayscale transition occurring from a previous frame to a current frame.
For example, in a situation where grayscale transition from a previous frame FR(k−1) to a current frame FR(k) is rise driving, a voltage higher than a voltage represented by video data D(i,j,k) of the current frame FR(k) is applied to pixels so that grayscale transition from the previous frame to the current frame is emphasized. More specifically, a voltage higher than a voltage represented by video data D(i,j,k) of the current frame FR(k) is applied to pixels.
As a result of this, when the grayscale transition occurs, the luminance level of the pixels more sharply increases and reaches close to a luminance level corresponding to the image data D(i, j, k) of the above-mentioned current frame FR(k) in a shorter time, as compared with a luminance level realized by a voltage level represented by video data D(i, j, k) of the current frame FR(k) being directly applied. Because of this, even in a case where a response speed of liquid crystal is slow, it is possible to improve a response speed of the liquid crystal display device.
Note that the following liquid crystal driving method is herein referred to as overshoot (OS) driving. That is, the liquid crystal driving method, as described in Patent document 1, is such that according to a combination of incoming image data of a current frame and incoming image data of a previous frame, a (overshot) drive voltage which is higher than a predetermined gray-scale voltage of the incoming image data of the current frame or a (undershot) drive voltage which is lower than the predetermined gray-scale voltage is supplied to a liquid crystal display panel.
Further, it is known that liquid crystal has variations in response speed depending upon environmental temperature. A response speed of liquid crystal is slow at low temperatures, in particular. For example, Patent document 2 (Japanese Unexamined Patent Publication No. 318516/1992 (Tokukaihei 4-318516); published on Nov. 10, 1992) suggests a liquid crystal panel driving device which emphasizes grayscale transition in accordance with a temperature.
Furthermore, for example, Patent document 3 (Japanese Unexamined Patent Publication No. 165087/1994 (Tokukaihei 6-165087); published on Jun. 10, 1994) discloses an arrangement in which a gain of a response speed compensation circuit which generates a correction voltage larger than the amount of change in video signal is adjusted in accordance with content of an image and user's preference, in order to provide a visible, high-definition liquid crystal display device which realizes noise removal from still images in MUSE (Multiple sub-Nyquist Sampling Encoding) signals, removal of line flicker, the rise of vertical resolution, smooth displays of moving images, and faithful high-speed displays for pan, tilt, scene change, and baseband signals.
Particularly, a driving method adopted in many liquid crystal display devices is a driving method such that in driving pixels in accordance with interlaced video signals, the interlaced video signals are converted into progressive video signals so that all of the pixels are driven by line-sequential scanning driving.
Referring to FIGS. 31 through 34, the following will describe details of a liquid crystal display device which performs overshoot driving to compensate for optical response properties of a liquid crystal display panel in accordance with its use environmental temperature. Here, FIG. 31 is a block diagram illustrating essential components of the conventional liquid crystal display device. FIG. 32. is a functional block diagram illustrating a schematic configuration of a control CPU. FIG. 33 is an explanatory diagram illustrating the relation between a device internal temperature and reference table memory. FIG. 34 is an explanatory diagram illustrating the relationship between a voltage applied to liquid crystal and a response of the liquid crystal.
In FIG. 31, reference numerals 501a through 501d represent OS table memories (ROMs) each of which stores OS parameter (emphasis conversion parameter) corresponding to every device internal temperature range. The OS parameter corresponds to grayscale transition between one frame and the previous or next frame of incoming image data. Reference numeral 515 represents frame memory (FM) which stores one frame of incoming image data. Reference numeral 514H represents an emphasis conversion section which compares incoming image data of Mth frame yet to be displayed (Current Data) with incoming image data of M−1th frame stored in the frame memory 515 (Previous Data), reads OS parameter corresponding to a result of the comparison (grayscale transition) from any of the OS table memories (ROMs) 501a through 501d, and determines emphasis conversion data (writing grayscale data) required for display of the image corresponding to the Mth frame in accordance with the thus read OS parameter.
Reference numeral 516 represents a liquid crystal controller which outputs a liquid crystal drive signal to a gate driver 518 and a source driver 519 of a liquid crystal display panel 517. Reference numeral 520 represents a temperature sensor for detecting a temperature inside the liquid crystal display device. Reference numeral 512H represents a control CPU which selects and references to any of the OS table memories (ROM) 501a through 501d in accordance with a device internal temperature detected by the temperature sensor 520, and outputs to the emphasis conversion section 514H a switch control signal for changing OS parameter for use in emphasis conversion of image data.
Here, OS parameters LEVEL1 through LEVEL4, which are stored in OS table memories (ROMs) 501a through 501d, respectively, are obtained in advance from actually measured values for optical response properties of the liquid crystal display panel 517 in an environment of reference temperatures T1, T2, T3, and T4 (T1<T2<T3<T4), respectively. In terms of the degree of emphasis conversion, there is the relationship of LEVEL1>LEVEL2>LEVEL3>LEVEL4.
The control CPU 512H, as illustrated in FIG. 32, has a threshold determination section 512a and a control signal output section 512b. The threshold determination section 512a compares temperature detection data obtained by the temperature sensor 520 with given threshold temperature data values Th1, Th2, and Th3 which are determined in advance. The control signal output section 512b selects any of the OS table memories (ROM) 501a through 501d according to a result of the comparison performed by the threshold determination section 512a, and generates a switch control signal for switching between OS parameters LEVEL1 through LEVEL4 and then outputs the generated switch control signal.
Here, for example, as illustrated in FIG. 33, when a device internal temperature detected by the temperature sensor 520 is equal to or lower than a switch threshold temperature Th1 (=15° C.), the control CPU 512H instructs the emphasis conversion section 514H to select and reference to the OS table memory (ROM) 501a. In response to the instruction, the emphasis conversion section 514H performs emphasis conversion processing on the incoming image data, by using the OS parameter LEVEL1 stored in the OS table memory (ROM) 501a. 
Further, when the device internal temperature detected by the temperature sensor 520 is higher than the switch threshold temperature Th1 (=15° C.) but not higher than a switch threshold temperature Th2 (=25° C.), the control CPU 512H instructs the emphasis conversion section 514H to select and reference to the OS table memory (ROM) 501b. In response to the instruction, the emphasis conversion section 514H performs emphasis conversion processing on the incoming image data, by using the OS parameter LEVEL2 stored in the OS table memory (ROM) 501b. 
Still further, when the device internal temperature detected by the temperature sensor 520 is higher than the switch threshold temperature Th2 (=25° C.) but not higher than a switch threshold temperature Th2 (=35° C.), the control CPU 512H instructs the emphasis conversion section 514H to select and reference to the OS table memory (ROM) 501c. In response to the instruction, the emphasis conversion section 514H performs emphasis conversion processing on the incoming image data, by using the OS parameter LEVEL3 stored in the OS table memory (ROM) 501c. 
Yet further, when the device internal temperature. detected by the temperature sensor 520 is higher than the switch threshold temperature Th3 (=35° C.), the control CPU 512H instructs the emphasis conversion section 514H to select and refer to the OS table memory (ROM) 501d. In response to the instruction, the emphasis conversion section 514H performs emphasis conversion process on the incoming image data, by using the OS parameter LEVEL4 stored in the OS table memory (ROM) 501d. 
Generally, a liquid crystal display panel requires a long time for change from one intermediate tone to another. This causes a extremely poor response to an incoming signal at low temperatures, thus increasing a response time. For this reason, the liquid crystal display panel has the problem that an intermediate tone cannot be displayed within one frame period (e.g. within 16.7 msc in progressive scanning of 60 Hz). This results in the occurrence of afterimage and a poorly produced halftone image. However, it is possible to display a target intermediate tone in a short time (within one frame period) as illustrated in FIG. 34, by using the aforesaid overshoot drive circuit to perform emphasis conversion of grayscale level of incoming image data in a grayscale transition direction so that the liquid crystal display panel 517 attains a target intermediate tone luminance defined by the incoming image data after an elapse of a predetermined one frame display period.
However, in a case where the display device can select an interlace-to-progressive conversion method from among a plurality of conversion methods, the arrangements of the Patent documents 1 through 3 have the problem of difficulty in preventing degradation of quality of video image.
More specifically, there is a wide variety of interlace-to-progressive conversion methods. The conversion method most suitable for all of the possible situations does not exist because it is determined depending upon a S/N ratio of incoming interlaced video signal, content of video image, user's preference, and others.
For example, there is a conversion method of performing interpolation between fields by motion detection and motion prediction compensation between adjacent fields. This method realizes improvement in quality of video image under conditions where a S/N ratio of an interlaced video signal is sufficiently high, as compared with a method like line doubling, i.e. a method of converting into a progressive video signal by copying a video signal of a horizontal line, which is a component of a certain field, supplied to pixels. The above conversion method, however, causes noise greater than the copying method under conditions where the S/N ratio is lower than an expected range of S/N ratio, thus resulting in degradation of quality of video image.
The use of the method like line doubling, i.e. the method of converting into a progressive video signal only by using data in a field decrease spatial resolution and thus reduces noise. However, this method has the problem that unwanted luminance change (flicker) is likely to occur in every frame, particularly, in edge portions of static image.
However, the arrangements of Patent documents 1 through 3 cannot emphasize grayscale transition appropriately in accordance with characteristics of interlace-to-progressive conversion. Thus, it might be possible that enhancement of the foregoing unwanted luminance change increase flickers on edge portions of a static image. This might seriously degrade quality of an image displayed.
More specifically, the I/P conversion processing, as illustrated in FIG. 35, both odd-numbered field and even-numbered field of an interlaced signal are subjected to data interpolation, and then, each of the odd-numbered field and even-numbered field is converted into image data which is one frame long, as illustrated in FIG. 36.
With this arrangement, an interlaced video signal (in case of NTSC broadcasting scheme) of 30 frames per second (60 fields per second) is converted into a quasi-progressive video signal of 60 frames per second. Thus, it is possible to make the interlaced video signal to be displayed as a progressive video signal.
However, suppose, for example, such an I/P conversion processing is the interpolation with only the respective data of even-numbered field and odd-numbered field in the interlaced scanning. As represented by dotted lines in FIG. 36, the I/P conversion processing causes variations in edge positions in a static image, which are supposed to be stationary, from field to field. Because of this, flicker noise (false signal) occurs, and jaggies of slanted lines (difference in brightness) appears.
Thus, suppose that interlaced signal with a sufficiently high S/N ratio is subjected to motion adaptive I/P conversion, or image data is subjected to emphasis conversion by the foregoing overshoot driving with a degree of emphasis that is the same as in the case where a progressive signal is supplied. Such an I/P conversion processing produces images emphasized with unwanted flicker noise (false signal) and jaggies of slanted lines (difference in brightness) caused by the I/P conversion processing, resulting in quality degradation of images displayed.